1. Field of the Invention
The present invention relates to restraining so-called chatter vibrations of a workpiece during machining, and more specifically, to a workpiece support method and a workpiece support device suitable for restraining chatter of a workpiece during machining.
2. Description of the Related Art
In machining such as cutting or grinding of a workpiece, a force applied from a tool to the workpiece sometimes causes so-called chatter of the workpiece. The chatter reduces accuracy of a machining size of the workpiece and increases roughness of a surface machined of the workpiece, and also sometimes makes machining itself difficult.
A workpiece having a simple shape can be easily secured or clamped onto a table of a machine tool, and the workpiece can be secured at a portion near a portion to be machined thereof if required and firmly secured so as to restrain chatter.
A workpiece having a complicated shape, however, cannot be always secured at a portion near a portion to be machined thereof. In this case, getting down a machining speed can restrain chatter to a certain extent, but decreases machining efficiency. Thus, there is a need for some measures to restrain chatter in order to keep high machining efficiency.
FIG. 1 shows an example of a conventional method for restraining chatter. This example shows a case of grinding an upper surface of one side of a thin box-shaped workpiece with an open upper side.
In FIG. 1, a workpiece W is made of metal and has the box shape with the open upper side, and a case is shown of, for example, grinding the upper surface S of one L of vertical walls by a tool T mounted to an unshown machine tool. The workpiece W is secured at a bottom wall B thereof to a table 1 of the machine tool by means of a main clamp 2 shown in a simplified manner. Reference numeral 3 denotes a workpiece support, which is secured onto the table 1. A shaft 4 of the workpiece support protrudes from a casing 5 and is secured, with a tip of the shaft 4 abutting against the vertical wall L of the workpiece W, at a portion near a portion to be machined of the workpiece W. Now, the support 3 will be simply described with reference to FIG. 2.
The casing 5 of the support 3 includes a substantially cylindrical casing body 6 having one open end and the other closed end and a lid 7 that closes the open end, and has a space 8 formed therein. Holes are coaxially drilled in a rear wall 6a of the casing body 6 and the lid 7, and the shaft 4 is provided movably in an axial direction through these holes. In the space 8, a collet 9 is placed around the shaft 4. The collet 9 abuts at both ends thereof against the rear wall 6a of the casing body 6 and the lid 7, and is not axially movable. An outer diameter of the collet 9 gradually increases toward the right in FIG. 2. Reference numeral 9a denotes a seat.
A cylindrical piston 10 is placed outside the collet 9 so as to surround the collet 9, and an inner diameter of the piston 10 gradually increases toward the right in FIG. 2 so as to be parallel to an outer periphery of the collet 9. The piston 10 has a flange 11 whose outer periphery comes into contact with an inner periphery of the housing body 6. Reference numeral 12 denotes a spring that urges the piston 10 to the left in FIG. 2. The housing body 6 has a hydraulic port 13, and a pressure fluid is supplied from a supply source P through the port 13 and acts on a left surface of the flange 11 of the piston 10 in the space 8 to move the piston 10 to the right in FIG. 2. When the piston 10 moves to the right, the piston 10 narrows the collet 9 to fasten the shaft 4 and secure the shaft 4 in that position.
When no hydraulic pressure acts, the piston 10 moves in the opposite direction, that is, to the left by the action of the spring 12 to loosen the collet 9 and release the shaft 4. Because the collet 9 does not fasten the shaft 4, the shaft can freely move in an axial direction. In FIG. 1, the workpiece W is secured by the main clamp 2, and the support 3 is also clamped and secured by an unshown appropriate clamp. Then, the shaft 4 is urged by an unshown spring to the left in FIG. 2, and the tip of the shaft 4 abuts against a side surface of the vertical wall L of the workpiece W. When a hydraulic pressure is supplied to the support 3 in this state, the piston 10 moves to the right in FIG. 2, and the collet 9 is narrowed to fasten the shaft 4, prevent the shaft 4 from moving in the axial direction, and secure the shaft 4 in that position.
In such a state where the tip of the shaft 4 of the support 3 abuts against the vertical wall L at a position near a portion C to be machined on the upper surface S of the workpiece W, the tool T is rotated for machining. At this time, the portion C being machined of the workpiece W tries to vibrate in directions shown by a double-headed arrow A in FIG. 2. It is intended to restrain the vibration by causing the shaft 4 to abut against the vertical wall L.
In the conventional restraining method, the shaft 4 is secured. Thus, when the workpiece W tries to vibrate from an initial position shown in FIG. 2 to the right of the double-headed arrow A under the action of the tool T, the shaft 4 prevents the movement of the workpiece W, and the tip of the shaft 4 keeps in contact with a portion near the portion C being machined of the workpiece W. Thus, the restraining method has an effect of restraining the vibration of the workpiece within a right-hand range. When the workpiece W vibrates to the left of the double-headed arrow A, however, the work piece W is separated from the tip of the shaft 4 because the shaft 4 is secured at the shown position, and the vibration cannot be restrained. Thus, the conventional method cannot sufficiently restrain so-called chatter vibrations, and has a problem in machining accuracy of a workpiece.
Vibrations of a workpiece that occur during machining by a tool are complicated, and primary order vibrations of relatively large amplitude of 5 micron to 10 micron to higher order vibrations of extremely small amplitude are superposed. The inventor of the invention has found that the higher vibrations of small amplitude among these vibrations have a large influence on finishing accuracy in machining a workpiece, and are significantly restrained by keeping a support member in contact with the workpiece with a contact pressure.